Strictly speaking, no one has consent. A husband does not require his wife's consent, and extramarital sex is adultery or fornication, and similarly illicit. My point about Lyanna is that she was not old enough to make her own decision about marriage. She was very likely a minor at the point she came into Rhaegar's company (whether willing or not), and had no lawful right to break her betrothal to Robert or to accept a proposal for a the fan-theorized polygamous marriage with marriage.